Certain types of fiber optic systems require couplers in which at least a portion of the light propagating in an optical fiber is coupled to one or more output fibers. The present invention relates to such fiber optic couplers and more particularly to a cost effective and reproducible method of making such fiber optic couplers.
It has been known that coupling occurs between two closely spaced cores in a multiple core device. The coupling efficiency increases with decreasing core separation and, in the case of single-mode cores, with decreasing core diameter. Couplers based on these principles have been developed for such uses as WDM couplers, star couplers, bend and Mach-Zehnder switches and power splitting couplers such as 3 dB couplers. Such couplers are capable of low loss operation; they typically exhibit an excess loss of about 1 dB or less.
Multimode and single-mode couplers have been formed by positioning a plurality of fibers in a side-by-side relationship along a suitable length thereof and fusing the claddings together to secure the fibers and reduce the spacings between the cores Coupling can be increased by stretching and by twisting the fibers along the fused length thereof as taught in U.S. Pat. No. 4,426,215 to Murphy; however, twisting the fibers is disadvantageous for certain purposes. Also, a portion of the cladding is sometimes removed by etching or grinding to decrease the intercore distance as taught in U.S. Pat. No. 4,449,781 to Lightstone et al. Since the coupling region is fragile and is exposed to the atmosphere, such couplers must then be provided with a hermetic enclosure. These processes are labor intensive and therefore expensive. Furthermore, they do not always result in couplers exhibiting predetermined desired coupling characteristics and long term integrity Such disadvantages are particularly apparent in the manufacture of certain single-mode couplers wherein the coupling core sections are to remain parallel to each other to insure that the propagation constants are matched and in the manufacture of certain single-mode couplers which must possess optical characteristics such as polarization retention.
An already formed coupler does not have to be immediately provided with an enclosure if the process of forming the coupler inherently hermetically seals and adequately supports the coupling of the fibers. However, attempts to accomplish this result have resulted in the formation of couplers having undesirable optical characteristics.
Japanese published application 60-140208 teaches a coupler formed by pre-twisting a pair of fibers, inserting them into a quartz tube, and heating and drawing the central part of the tube to reduce its diameter. Resin is then applied to the ends of the tube to seal the fibers thereto. This coupler has the following disadvantages. During the collapse of the tube onto the fibers, the fibers are not held taut and the capillary tube is not evacuated. The fibers are therefore free to meander in the tube, thereby preventing the achievement of a predetermined coupling when the tube is elongated by a predetermined length. This can also make it difficult to achieve low coupler loss. Since the fibers are pre-twisted in order to provide a sufficient length of fiber-to-fiber contact to provide adequate coupling, the resultant coupler cannot maintain the polarization of an input optical signal and is unsuitable for certain applications.
U.S. Pat. No. 3,579,316 to Dyott et al. teaches a method wherein the fibers are first inserted into a capillary tube where the ends may overlap. The capillary tube is formed of a glass having a refractive index lower than that of the fiber cladding material. Heat is applied to the capillary tube in the vicinity of the fiber overlap, and the tube is stretched until the diameter thereof approximates that of the original fibers. The original cores of the pulled out part become vanishingly small, their stretched diameters being only about 1/100 the original diameters; the cladding of the original fibers becomes the core of the coupling section. In a second embodiment, wherein there is a partial transfer of power from one fiber to the other. The two fibers are inserted into the tube with their ends extending from both tube ends. The bore of the capillary tube is sufficiently large that one of the fibers is surrounded by a sleeve having the same refractive index as the capillary tube. Since the cores become vanishingly small and the original claddings become the cores of the coupler section, the sleeve keeps the new cores a fixed distance apart. Such a long thin coupler is very cumbersome and fragile. Furthermore, such a coupler is lossy since the original cladding takes the place of the vanished cores. In the region of the coupler where the fiber cores taper from their "vanishingly small" size to their full size, an insufficient amount of power is transferred from the cladding back to the core.
Various disadvantages of the aforementioned prior art have been overcome by the method of said related application Ser. No. 204,620 wherein at least two suitably prepared glass optical fibers, each having a core and cladding, are disposed within the longitudinal aperture of a glass tube in a manner such that the ends of the fibers extend beyond the ends of the tube. If the glass fibers have a coating thereon, a portion of the coating intermediate the ends thereof is removed, the uncoated portion of the glass fibers being disposed within the longitudinal aperture of the tube. The fibers are held taut and are glued to the first and second end portions of the tube; after the glue has cured, the fibers remain in tension. The midregion of the assembly so formed is heated to cause it to collapse around the fibers, and it is drawn down to a predetermined diameter. Collapse of the tube is facilitated by creating a lower pressure within the aperture.
As the central portion the glass tube collapses, the aperture decreases in size, and the tube contacts the fibers, thereby tending to force them into mutual contact. This occurrence increases the probability that the resultant coupler will possess predetermined coupling characteristics. However, glass from the collapsing tube can also flow into the region between the fibers. If this flow predominates, it can cause a separation between the fibers which affects coupling.